My sister is 16 years old and my sister is 10 years old. After _ _ _ years, my sister and younger sister are 40 years old.

No matter how many years have passed, the age difference between elder sister and younger sister will not change. Their age difference is 16- 10=6 (years old), so that this problem can be transformed into a "sum difference problem", so as to find out the age of elder sister or younger sister when the sum of the two sisters' ages is 40, and then find out how many years have passed. 1. When the sisters are 40 years old, the elder sister's age is: (16-10+40) ÷ 2 = 46 ÷ 2 = 23 (year) 23- 16=7 (year Note that the age difference between brother and sister will not change, which is the key to solve this problem.